The paper presents the CFD model and numerical results of combustion process simulations in a two-stroke, uniflow scavenging dual free piston engine, FPE, designed for electricity generation. Two fuels, diesel oil and dimethyl ether (DME),were studied in order to achieve HCCI-like combustion. It is illustrated that by varying the direc injection timing, a conparably efficient, low emission operation has been predicted for both fuel.